Aerospace components made of superalloys such as nickel and cobalt-based superalloys are susceptible to oxidation, reducing their service life and necessitating their replacement or repair. For example, gas turbine engine components such as, for example, the burner assembly, turbine vanes, nozzles, and blades are susceptible to oxidation because they encounter severe operating conditions at high temperature conditions. As used herein, “severe operating conditions” include high gas velocities and exposure to salt, sulfur, and sand causing hot corrosion or erosion and “high temperature conditions” refers to temperatures of about 700° C. to about 1150° C. The oxidation resistance of such superalloy components can be enhanced by applying protective coatings.
Simple aluminide coatings are used on superalloy components to improve oxidation resistance, especially when cost is an issue. Platinum aluminide coatings are used in even more demanding applications. There are several drawbacks to conventional aluminum deposition techniques. For example, chemical vapor deposition (CVD) is costly and requires using dangerous gases. While deposition using pack cementation is less costly, there are also drawbacks associated with this conventional deposition technique, such as the introduction of impurities into the aluminum, thereby reducing coating life. For both of these gaseous aluminizing processes, the temperatures used are high so that the aluminum diffuses into the superalloy substrate/component as it is deposited such that the surface aluminide is only about 20-30% aluminum. There are lower temperature aluminum CVD deposition processes that do not result in aluminum diffusion, but these processes are only used in a few specialized applications, because of the dangerous gases involved. In addition, as CVD and pack cementation deposition processes are performed at high temperatures, under aggressive deposition conditions, high cost masking techniques prior to deposition are used to ensure that high stress areas of the superalloy component are not coated. After deposition or coating, the masks are removed. High temperature (and high cost) masking techniques include applying masking pastes to the component by spraying or dipping. Extreme care (and labor) has to be taken to ensure that only the desired areas are coated. These pastes form hard deposits that are difficult and labor intensive to remove.
Aluminum electroplating processes may also be used to deposit aluminum at high purity levels, but conventional aluminum electroplating is complex, costly, performed at high temperatures, and/or requires the use of flammable solvents and pyrophoric compounds, which decompose, evaporate and are oxygen-sensitive, necessitating costly specialized equipment and presenting serious safety and environmental challenges to a commercial production facility. In addition, for all aluminum electroplating processes on superalloys, the aluminum is present after plating as an aluminum layer on the surface of the substrate. The aluminum layer needs bonding and diffusion into the superalloy component to produce a high temperature oxidation resistant aluminide coating. As used herein, the term “aluminide coating” refers to the coating after diffusion of aluminum into the superalloy component. If conventional aluminum diffusion temperatures of 1050° C. to 1100° C. are used, undesirable microstructures are created. In addition, as conventional diffusion into a superalloy component causes its embrittlement reducing its life, great care has to be taken to ensure that high stress areas are not coated using high temperature masking techniques as previously described.
Ionic liquids have been used to deposit aluminum on non-superalloy substrates for corrosion and wear and tear resistance in a lab-scale three-step process that includes a first pretreatment step in which the substrate is cleaned, degreased, pickled, and then dried. In the second step, the metal substrate is then electroplated using the ionic liquid at a temperature ranging from 60 to 100° C. The third step includes removing the ionic liquid from the substrate.
It is well established that small additions of the so-called “reactive elements” (R.E.) such as silicon, hafnium, zirconium, cerium, and lanthanum increase the oxidation resistance of high temperature aluminide coatings. Unfortunately, the co-deposition of aluminum and the reactive element is difficult, expensive, and can be dangerous. In a best case scenario, the co-deposit requires at least two separate deposition processes, such as the initial deposit of aluminum by a chemical vapor deposition process, pack cementation process, or the like followed by deposition of the reactive element by another chemical vapor deposition process in the same or a different reactor. A heat-treated slurry coating containing aluminum and hafnium particles has also been used in an attempt to co-deposit aluminum and hafnium to form a protective aluminide-hafnium coating, but the results have been disappointing with the hafnium particles not sufficiently diffusing into the aluminum, the base metal of the coated component oxidizing, and the concentration of the reactive element unable to be controlled.
Accordingly, it is desirable to provide methods for producing a high purity, high temperature oxidation resistant coating on superalloy components, including gas turbine engine components. In addition, it is desirable to provide methods for producing a high temperature oxidation resistant coating on a superalloy component using a simplified, lower cost, safe, and environmentally-friendly method including the use of low temperature masking techniques. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.